Polyamide resins such as polyamide 6.6 and polyamide 6 are very strong resins well suited for molding of various articles; however, such resins are sensitive to moisture. To reduce moisture pick-up, polyolefins such as polypropylene or polyethylene are added to the polyamide. However, due to the polar nature of polyamides and the non-polar nature of polyolefins, blending of such resins leads to compatibility problems.
Physical melt blending of a polyamide and a polyolefin does not result in acceptable end-use properties due to insufficient compatibility. This results in severe delamination particularly in injection molded articles, and moisture absorption is actually increased compared to the pure polyamide.
It is known that in order to improve the compatibility of polyamides and polyolefins, the addition of a modified polyolefin that is able to interact with the end-groups of the polyamide is necessary. The modification of polyolefins is obtained by, for example, reacting it with an unsaturated carboxylic acid, such as fumaric acid for example, or the anhydride thereof, such as maleic anhydride, as described, for example, in European Patent Application 370,736. Various compositions of grafted polyolefin compatibilized blends of polyamide and polyolefins are known and the patent disclosure in this area is plentiful.
In order to enhance compatibility between a polyamide and a polyolefin such as polypropylene or polyethylene, it has been suggested to either add the modified polyolefin to a blend of polyamide and unmodified polyolefin or to blend only modified polyolefin with the polyamide.
The general disadvantage of thus compatibilized blends is their relatively low overall toughness. Pat. No ES 8,305,395 to Du Pont describes a combination of a polyamide and a maleic acid or anhydride grafted polyethylene which may generally cover the use of a grafted polyolefin as compatibilizer for polyamide/polyolefin blends.
For improving impact strength of polyamide/polyolefin blends, it has been suggested to use both a modified polyolefin such as polypropylene for example and a modified elastomer or to use only a modified elastomer. While the use of only a modified elastomer does improve impact properties somewhat, it usually yields resins with anisotropic properties due to insufficient homogeneity and a laminar type morphology. The use of higher concentrations of modified elastomer can improve morphology somewhat, but at the expense of a significant reduction in flexural modulus. As to the use of both, i.e. modified polyolefin and modified elastomer, it has been proposed to either simultaneously modify polyamide, unmodified polypropylene and elastomer (Japanese patent HEI 1(1989)-103662), to simultaneously modify polypropylene and elastomer (EP 194-705-A and EP 235-876-A, for example) or to separately modify polypropylene and elastomer (Jap. 60053-550-A).
However, while the compatibilization and toughness of polyamide and polyolefin can be improved effectively by the use of such modified polyolefins and modified elastomers, `macroscopic morphology` in injection molded articles of those kinds of blends is still insufficient, which is even more so in blends which are richer in polyamide than in polyolefin. The term `macroscopic morphology` refers to the perceived integrity and appearance of injection molded parts throughout the cross section of an injection molded article, opposed to `microscopic morphology` which is usually determined by Transmission Electron Microscopy (TEM). Experience has shown that, while compatibilized blends of polyamide and polyolefin can exhibit very good `microscopic morphology`, with the polyolefin phase finely dispersed within a continuous polyamide phase, and with a very high surface gloss, the `macroscopic morphology` of the same blend can be very poor, as manifested by delamination of the complete surface layer of molded articles. This kind of delamination appears to be more severe for parts with a large surface area and a radial flow pattern; however, delamination of the surface layer with such poorly compatibilized blends can also be seen in the sprue and runner system of smaller parts. The chemical composition of the blend in the delaminated skin and the remaining core section is typically shown to be identical when analyzed by Infrared Spectroscopy, Differential Scanning Calorimetry and Scanning Electron Microscopy.
Polyamide/polyolefin blends as claimed in EP 235-876-A, for example, when applied to blends which are rich in polyamide, do not yield blends with a good `macroscopic morphology`, since it is recommended there that the polyamide needs to be more highly viscous than the unmodified and the modified polyolefin. Experience has further shown that neither improved gloss nor reduced discoloration are at all conclusive evidence for improved compatibility. Many polyamide/polyolefin blends have been made with excellent gloss and color, yet the `macroscopic morphology` as described above was still insufficient. The problem is not so much the appearance of the visible outer surface rather than the adhesion of the often very homogeneous outer surface layer to the core material. To applicant's knowledge this issue has not been addressed in the prior art to polyamide/polyolefin blends claimed in this invention.
It has now been found that, in order to eliminate the tendency for delamination of the outer surface layer of injection molded articles in polyamide/polyolefin blends, the polyamide must be the continuous phase. This can be achieved by keeping the melt viscosity of the polyamide sufficiently low relative to the viscosity of the unmodified polyolefin and preferably the modified compatibilizers as well. Only then will the polyamide form the continuous phase throughout the whole cross section of injection molded articles, as long as sufficient shear mixing is provided during extrusion and subsequent injection molding.